The present invention relates to a method for operating an internal combustion engine, in particular of a motor vehicle, in which a setpoint fuel mass and/or a setpoint air mass is/are determined as a function of a setpoint torque, and in which a transition is made from a first operating mode to a second operating mode of the internal combustion engine. The present invention also relates to a corresponding internal combustion engine as well as to a corresponding control unit for an internal combustion engine.
In the case of gasoline engines as well as diesel gasoline engines, it is known to use so-called adsorption catalysts to reduce nitrogen oxides (NOx) produced during combustion. These adsorption catalysts are suitable for temporarily storing the nitrogen oxides, particularly during a first lean operating mode of the internal combustion engine, in order to then release them again and reduce them in a subsequent second rich operating mode of the internal combustion engine. This transition to the rich operating mode necessary for regenerating the adsorption catalyst represents a problem. In particular, it must be ensured that this transition is carried out smoothly. Therefore, during the transition, the actual torque of the internal combustion engine may not or may only slightly deviate from the setpoint torque.
The actual torque can be measured with the aid of a torque sensor. It is also known in accordance with German Published Patent Application No. 44 45 684 to determine the actual torque as a function of the rotational speed of the internal combustion engine.
It is an object of the present invention to create a method for operating an internal combustion engine, which enables a smooth transition to the second operating mode of the internal combustion engine and, thus, to the regeneration of the adsorption catalyst.
According to the present invention, in the case of a method of the species cited at the outset, this object is achieved in that an actual torque is determined during the transition to the second operating mode of the internal combustion engine, a torque difference between the setpoint torque and the actual torque is calculated, and in that the setpoint fuel mass and/or the setpoint air mass is/are controlled as a function of the torque difference.
As mentioned, determining the setpoint fuel mass and/or setpoint air mass as a function of the setpoint torque can lead to comfort problems during the transition to the regeneration of the adsorption catalyst. In this context, the comfort problems are the result of the actual torque deviating from the desired setpoint torque. Such a deviation is detected by comparing the actual torque and the setpoint torque in accordance with the present invention. According to the present invention, the indicated torque difference is limited to a minimum by subsequently influencing the setpoint fuel mass and/or the setpoint air mass as a function of the determined torque difference. Finally, the present invention makes it possible to immediately detect any deviations in torque and to take appropriate countermeasures. The torque deviations can, thus, be kept to a minimum. At the same time, this represents an increase in comfort during the transition to the rich operating mode of the internal combustion engine and, thus, during the regeneration of the adsorption catalyst.
In this context, it is significant that a torque sensor is not required for determining the torque difference in accordance with the present invention. Instead, as already mentioned, it is also possible to derive the actual torque from the rotational speed of the internal combustion engine, as an absolute quantity. Thus, no additional sensor or the like is needed to carry out the present invention. At the same time, this represents a reduction in cost and constructive effort.
It is particularly advantageous when the actual torque is determined from the rotational speed of the internal combustion engine, as a relative quantity. If the setpoint torque is also determined as a relative quantity, comparing these two quantities yields the same result as comparing the corresponding absolute quantities. However, using the relative quantities results, in particular, in the significant advantage that the change in the actual torque can be easily derived from the change in the rotational speed of the internal combustion engine.
It is particularly advantageous-when the torque difference is adjusted to zero by a controller. Using such a controller makes it possible to achieve optimum driving comfort. With the aid of the controller, jolting during the transition to the regeneration of the adsorption catalyst can be completely prevented.
In an advantageous embodiment of the present invention, the setpoint fuel mass and/or the setpoint air mass is/are controlled by the torque difference using a controller action model. In this manner, it is taken into consideration that the method of the present invention is essentially only used during the transition to the rich operating mode of the internal combustion engine and, thus, during the transition to the regeneration of the adsorption catalyst. It is possible via the controller action model to reliably prevent any losses of comfort due to jolting or the like when transitioning to the regeneration of the adsorption catalyst.
In an advantageous further refinement of the present invention, the setpoint fuel mass and/or the setpoint air mass is/are controlled by a control system as a function of a setpoint torque. This means that during normal operation of the internal combustion engine, the setpoint fuel mass and/or the setpoint air mass is/are not regulated, but is/are only controlled as a function of the setpoint torque. This forward control is generally completely sufficient during a lean operating mode of the internal combustion engine. Only during a rich operating mode of the internal combustion engine, i.e., during the regeneration of the adsorption catalyst among other things, this control can lead to inaccuracies and, thus, to torque deviations as explained at the outset. These torque deviations can then also be compensated for or corrected by the present invention.
In a particularly advantageous embodiment of the present invention, instead of the setpoint fuel mass and/or the setpoint air mass, the setpoint injection interval between a first partial injection and a second partial injection is controlled as a function of the torque difference.
This embodiment of the present invention is particularly advantageous especially when the fuel/air mixture supplied to the combustion chamber of the internal combustion engine already has the desired lambda. Changing the setpoint injection interval between the first and the second partial injection does not change this lambda. Instead, changing the setpoint injection interval only affects the actual torque generated by the internal combustion engine. In this particularly advantageous manner, the actual torque can be easily adapted to the setpoint torque without thereby changing the lambda of the internal combustion engine and, thus, the exhaust-gas composition of the internal combustion engine.
Particularly significant is the implementation of the method of the present invention in the form of a control element provided for a control unit of an internal combustion engine, in particular of a motor vehicle. In this context, a program that can be executed on a computing element, in particular on a microprocessor, and is suitable for carrying out the method according to the present invention is stored on the control element. Thus, in this case, the present invention is implemented by a program stored on the control element, so that this control element provided with the program represents the present invention in the same manner as the method for whose implementation the program is suited. In particular, an electrical storage medium, e.g., a read only memory or a flash memory, can be used as the control element.